Due to the heavy use of wireless communications in both civilian and military environments, there is often a shortage of available communication bandwidth. Traditionally, frequency “bands” have been assigned on a regional or global basis according to specific types of usage, such as commercial AM and FM radio, commercial VHF and UHF television, citizens band radio, licensed amateur radio, cellular telephony, satellite communication, ship-to-shore communication, aviation communication, military communication, and such like. Within many of these bands, such as commercial television and radio bands, specific frequencies or “channels” are assigned to individual entities, such as channels assigned to specific radio and television stations. Typically, such assignments provide for exclusive use of the assigned channel over a designated geographic region.
This traditional approach of exclusively reserving specific communication channels for specific entities generally leads to inefficient use of bandwidth, since at any given time, and in any given location, it is unlikely that all of the assigned channels will be in use. For example, a commercial television station may not have any broadcast coverage in certain portions of its assigned geographic region, and/or may broadcast only at certain times, leaving the assigned channel empty and unused at other locations and/or at other times.
One approach to taking advantage of this unused bandwidth is to use “Cognitive Radio” or “CR” technology. A cognitive radio is a radio that is capable of sensing its local bandwidth environment, so as to determine at any given time what frequencies are unused (so-called “white spaces”) or underused (so-called “grey spaces”). Cognitive radios can then opportunistically use these white and/or grey spaces to communicate with each other without requiring a fixed, dedicated frequency assignment. It is fundamental to this approach that the cognitive radios function as secondary users of whatever channels they select. Therefore, they must effectively monitor the channels at all times for primary, or “incumbent” usage, and avoid any interference with the incumbents.
In particular, with reference to FIG. 1, cognitive radios can be used to form a so-called “Wireless Regional Area Network,” or WRAN. In a WRAN, a base station CR 100 communicates with a plurality of subscriber CR's, or “subscribers” 102. In some implementations where the subscribers are not mobile, they are referred to as “Customer Premise Equipment” or “CPE” CR radios. The base station 100 determines which channels are available at any given time and communicates with the subscribers 102 to direct the usage thereof by the WRAN. In the simplest case, the base station 100 monitors and analyzes the bandwidth environment, selects an available channel, and broadcasts information on that channel to the subscriber radios 102. The base station 100 may also consult a database of known incumbents 104 and their assigned channels, regions, and patterns of usage. When a subscriber radio 102 wishes to join the WRAN, it surveys the local bandwidth environment until the base station 100 is located, and then identifies itself to the base station 100 and joins the network. Once the WRAN is established, the base station 100 coordinates switching of the WRAN to other frequencies from time to time, if and as needed. In FIG. 1, the incumbent 104 is illustrated as not presently broadcasting, thereby leaving its assigned channel free for use by the WRAN.
With reference to FIG. 2, the network architecture of a WRAN is a so-called “star” configuration, whereby communication within the network is always between the base station 100 and a subscriber 102.
As illustrated in FIG. 3, the WRAN of FIG. 1 is easily able to avoid interference with powerful incumbents 104 such as television or radio stations, which can be reliably detected by the base station 100, and/or which have well established channels, regions, and patterns of usage that are available to the base station through consultation with an appropriate database.
This approach is insufficient, however, because an incumbent source may be operating somewhere within the region served by the WRAN without being detectable by the base station, and without being documented in an available database. Not all incumbents transmit with high power. For example, an incumbent may be a television news van transmitting a new “feed” to a relay station, or a wireless microphone transmitting signals to a nearby amplifier. Such low power incumbents can be active within an area covered by a WRAN and yet be out of range of the base station, or shielded from the base station by an intervening building or hill. This is sometimes called the “hidden node” problem. An example of a wireless microphone 400 “hidden node” is illustrated in FIG. 4, where the microphone 400 is within range of a nearby subscriber 102 but not of the base station 100. An example of a television news van 500 is illustrated in FIG. 5, where the news van 500 is in range of several subscribers 102, but is blocked from detection by the base station 100 by an intervening building 502.
So as to avoid the hidden node problem, “collaborative sensing” is typically employed in a WRAN, whereby the subscribers 102 monitor their local broadcast environments and report their findings to the base station 100. In a typical collaborative sensing implementation, the base station 100 then avoids any frequency channel on which at least one subscriber 102 has detected a signal. This is sometimes called the “OR” rule of collaborative detection. As can be seen in FIG. 4, the wireless microphone 400 is easily detected by the nearest subscriber 102, which then informs the base station 100 not to use the channel being used by the wireless microphone 400. In some embodiments, certain incumbents such as wireless microphones use a “co-existent beaconing protocol” or “CBP” to transmit a “beacon” signal including a digital certificate that can be verified by the WRAN so as to increase the confidence with which it is detected. One example of the CBP approach is described in proposed IEEE standard 802.22.1. In some embodiments, the digital certificate is an elliptic curve cryptography or ECC-based implicit certificate.
With reference to FIG. 5, although the mobile television news van 500 is not known a priori to the base station, and is blocked from detection by the base station 100 due to an intervening building 502, collaborative detection by nearby subscribers 102 enables the WRAN to overcome the “hidden node” problem.
Nevertheless, while collaborative sensing is helpful for addressing the hidden node problem, it can have the unintended result that available frequency channels are mistakenly judged to be in use by incumbents, and are avoided by the WRAN when they could otherwise be used. For example, with reference to FIG. 4, a weak signal unintentionally emitted by a computer 402 or other electronic device may be detected by a nearby subscriber 102 and misinterpreted as a valid incumbent, thereby causing an available channel to be avoided. If the unintentional signal has a wide bandwidth, this unnecessary avoidance of frequency channels may have a significant impact on the operation of the WRAN.
In addition, under malicious circumstances, a conventional WRAN can be highly vulnerable to a denial of service or “DoS” attack, since a single, concealed, low power jamming source within detection range of even a single subscriber can easily inhibit transmission over the WRAN by creating the false impression that all frequency channels are occupied by incumbents and are therefore not available. An example is illustrated by FIG. 6, which illustrates a hidden, low power transmitter 600 deliberately broadcasting signals that are detectable by a nearby cognitive radio (CR3) 102 and cause the WRAN to falsely conclude that all frequency channels are fully occupied.
What is needed, therefore, is a method for avoiding interference by a WRAN with incumbent sources in a cognitive radio network, while at the same time minimizing the vulnerability of the WRAN to spurious signals and denial of service attacks.